Fish locomotion
The
prevailing type of fish locomotion is swimming in water. In addition, some fish
can "walk", i.e., move over land, burrow in mud, and glide through
the air.
Swimming
Fish swim by
exerting force against the surrounding water. There are exceptions, but this is
normally achieved by the fish contracting muscles on either side of its body in
order to generate waves of flexion that travel the length of the body from nose
to tail, generally getting larger as they go along. The vector forces exerted
on the water by such motion cancel out laterally, but generate a net force
backwards which in turn pushes the fish forward through the water.
Most fishes
generate thrust using lateral movements of their body and caudal fin. But there
are also a huge number of species that move mainly using their median and
paired fins. The latter group profits from the gained maneuverability that is
needed when living in coral reefs for example. But they can't swim as fast as
fish using their bodies and caudal fins.
Body/caudal
fin propulsion
There are
five groups that differ in the fraction of their body that is displaced
laterally:
Anguilliform
locomotion
In some
long, slender fish – eels, for example – there is little increase in the
amplitude of the flexion wave as it passes along the body.
Sub-carangiform
locomotion
Here, there
is a more marked increase in wave amplitude along the body with the vast
majority of the work being done by the rear half of the fish. In general, the
fish body is stiffer, making for higher speed but reduced maneuverability.
Trout use sub-carangiform locomotion.
Carangiform
locomotion
Fish in this
group are stiffer and faster-moving than the previous groups. The vast majority
of movement is concentrated in the very rear of the body and tail. Carangiform
swimmers generally have rapidly oscillating tails.
Thunniform
locomotion
The
next-to-last group is reserved for the high-speed long-distance swimmers, like
tuna (new research shows that the thunniform locomotion is an autapomorphy of
the tunas). Here, virtually all the lateral movement is in the tail and the
region connecting the main body to the tail (the peduncle). The tail itself
tends to be large and crescent shaped.
Ostraciiform
locomotion
These fishes
have no appreciable body wave when they employ caudal locomotion. Only the tail
fin itself oscillates (often very rapidly) to create thrust. This group
includes Ostraciidae.
Median/paired
fin propulsion
Not all fish
fit comfortably in the above groups. Ocean sunfish, for example, have a
completely different system, and many small fish use their pectoral fins for
swimming as well as for steering and dynamic lift. Fish with electric organs,
such as those in Gymnotiformes, swim by undulating their fins while keeping the
body still, presumably so as not to disturb the electric field that they
generate.
Dynamic
lift
Bone and muscle tissues of fish are
denser than water. To maintain depth some fish increase buoyancy by means of a
gas bladder or by storing oils or lipids. Fish without these features use
dynamic lift instead. It is done using their pectoral fins in a manner similar
to the use of wings by airplanes and birds. As these fish swim, their pectoral
fins are positioned to create lift which allows the fish to maintain a certain
depth.
Sharks are a
notable example of fish that depend on dynamic lift; notice their
well-developed pectoral fins.
The two
major drawbacks of this method are that these fish must stay moving to stay
afloat and that they are incapable of swimming backwards or hovering.
Hydrodynamic
principles
Similarly to
the aerodynamics of flight, powered swimming requires animals to overcome drag
by producing thrust. Unlike flying, however, swimming animals do not
necessarily need to actively exert high vertical forces because the effect of
buoyancy can counter the downward pull of gravity, allowing these animals to
float without much effort. While there is great diversity in fish locomotion,
swimming behavior can be classified into two distinct "modes" based on
the body structures involved in thrust production, Median-Paired Fin (MPF) and
Body-Caudal Fin (BCF). Within each of these classifications, there are a
numerous specifications along a spectrum of behaviours from purely undulatory
to entirely oscillatory based. In undulatory swimming modes thrust is produced
by wave-like movements of the propulsive structure (usually a fin or the whole
body). Oscillatory modes, on the other hand, are characterized by thrust
production from swiveling of the propulsive structure on an attachment point
without any wave-like motion.
Median-paired
fin
Many fish swim using combined
behavior of their two pectoral fins or both their anal and dorsal fins.
Different types of Median Paired Fin (MPF) gait can be achieved by
preferentially using one fin pair over the other, and include
Undulatory
Rajiform:
seen in rays, skates, and mantas when thrust is produced by vertical
undulations along large, well developed pectoral fins.
Diodontiform:
in which propulsion is achieved by propagating undulations along large pectoral
fins
Amiiform:
undulations of a long dorsal fin while the body axis is held straight and
stable
Gymnotiform:
undulations of a long anal fin, essentially upside down amiiform
Balistiform:
both anal and dorsal fins undulate
Oscillatory
Tetradontiform:dorsal
and anal fins are flapped as a unit, either in phase or exactly opposing one
another. The ocean sunfish is an extreme example of this form of locomotion.
Labriform:
oscillatory movements of pectoral fins and can be classified as drag based or
lift based in which propulsion is generated either as a reaction to drag
produced by dragging the fins through the water in a rowing motion or via lift
mechanisms.
File:Sardines.ogg
Sardines use
body-caudal fin propulsion to swim and hold their pectoral, dorsal, and anal
fins flat against the body, creating a more streamlined body and reducing drag.
Body-caudal
fin
Most fish
swim by generating undulatory waves that propagate down the body through the
caudal fin. This form of undulatory locomotion is termed Body-Caudal Fin (BCF)
swimming on the basis of the body structures used.[3][4]
Undulatory
Anguilliform:
seen in eels and lampreys, this locomotion mode is marked by whole body in
large amplitude wavelengths. Both forward and backward swimming is possible in
this type of BCF swimming.
Subcarangiform:
similar to anguilliform swimming, but with limited amplitude anteriorly that
increases as the wave propagates posteriorly, this locomotion mode is often
seen in trout.
Carangiform:
body undulations are restricted to the posterior third of body length with
thrust produced by a stiff caudal fin
Thunniform:
the most efficient aquatic locomotion mode with thrust being generated by lift
during the lateral movements occurring in the caudal fin only. this locomotion
mode has evolved under independent circumstances in teleost (ray-finned) fish,
sharks, and marine mammals.
Oscillatory
Ostraciiform:
the body remains rigid and the stiff caudal fin is swept in a pendulum-like
oscillation. Fish using this type of BCF locomotion, usually rely predominantly
on MPF swimming modes, with ostraciiform behavior only an auxiliary behavior.
Adaptation
Similar to
adaptation in avian flight, swimming behaviors in fish can be thought of as a
balance of stability and maneuverability.[5] Because BCF swimming relies on
more caudal body structures that can direct powerful thrust only rearwards,
this form of locomotion is particularly effective for accelerating quickly and
cruising continuously.[3][4] BCF swimming is, therefore, inherently stable and
is often seen in fish with large migration patterns that must maximize
efficiency over long periods. Propulsive forces in MPF swimming, on the other
hand, are generated by multiple fins located on either side of the body that
can be coordinated to execute elaborate turns. As a result, MPF swimming is
well adapted for high maneuverability and is often seen in smaller fish that
require elaborate escape patterns.[5]
It is
important to point out that fish do not rely exclusively on one locomotor mode,
but are rather locomotor "generalists,"[3] choosing among and
combining behaviors from many available behavioral techniques. In fact, at
slower speeds, predominantly BCF swimmers will often incorporate movement of
their pectoral, anal, and dorsal fins as an additional stabilizing mechanism at
slower speeds, but hold them close to their body at high speeds to improve
streamlining and reducing drag.[3] Zebrafish have even been observed to alter
their locomotor behavior in response to changing hydrodynamic influences throughout
growth and maturation.
In addition
to adapting locomotor behavior, controlling buoyancy effects is critical for aquatic
survival since aquatic ecosystems vary greatly by depth. Fish generally control
their depth by regulating the amount of gas in specialized organs that are much
like balloons. By changing the amount of gas in these swim bladders, fish
actively control their density. If they increase the amount of air in their
swim bladder, their overall density will become less than the surrounding
water, and increased upward buoyancy pressures will cause the fish to rise
until they reach a depth at which they are again at equilibrium with the
surrounding water. In this way, fish behave essentially as a hot air balloon
does in air.
Flying
The
transition of predominantly swimming locomotion directly to flight has evolved
in a single family of marine fish called Exocoetidae. Flying fish are not true
fliers in the sense that they do not execute powered flight. Instead, these
species glide directly over the surface of the ocean water without ever
flapping their "wings." Flying fish have evolved abnormally large
pectoral fins that act as airfoils and provide lift when the fish launches
itself out of the water. Additional forward thrust and steering forces are
created by dipping the hypocaudal (i.e. bottom) lobe of their caudal fin into
the water and vibrating it very quickly, in contrast to diving birds in which
these forces are produced by the same locomotor module used for propulsion. Of
the 64 extant species of flying fish, only two distinct body plans exist, each
of which optimizes two different behaviors.
remains
of a flying fish are displayed in glass box.
Flying fish are able to achieve
sufficient lift to glide above the surface of the water thanks to their
enlarged pectoral fins.
Tradeoffs
Tail
Structure: While most fish have caudal fins with evenly sized lobes (i.e.
homocaudal), flying fish have an enlarged ventral lobe (i.e. hypocaudal) which
facilitates dipping only a portion of the tail back onto the water for
additional thrust production and steering.
Larger mass:
Because flying fish are primarily aquatic animals, their body density must be
close to that of water for buoyancy stability. This primary requirement for
swimming, however, means that flying fish are heavier than other habitual
fliers, resulting in higher wing loading and lift to drag ratios for flying
fish compared to a comparably sized bird. Differences in wing area, wing span,
wing loading, and aspect ratio have been used to classify flying fish into two
distinct classifications based on these different aerodynamic designs.
Biplane
body plan
In the
biplane or cypselurus body plan, both the pectoral and pelvic fins are enlarged
to provide lift during flight. These fish also tend to have "flatter"
bodies which increase the total lift producing area thus allowing them to
"hang" in the air better than more streamlined shapes. As a result of
this high lift production, these fish are excellent gliders and are well
adapted for maximizing flight distance and duration.
Comparatively,
Cypselurus flying fish have lower wing loading and smaller aspect ratios (i.e.
broader wings) than their Exocoetus monoplane counterparts, which contributes
to their ability to fly for longer distances than fish with this alternative
body plan. Flying fish with the biplane design take advantage of their high
lift production abilities when launching from the water by utilizing a
"taxiing glide" in which the hypocaudal lobe remains in the water to
generate thrust even after the trunk clears the water's surface and the wings
are opened with a small angle of attack for lift generation.
Monoplane
body plan
In the Exocoetus or monoplane body
plan, only the pectoral fins are enlarged to provide lift. Fish with this body
plan tend to have a more streamlined body, higher aspect ratios (long, narrow
wings), and higher wing loading than fish with the biplane body plan, making
these fish well adapted for higher flying speeds. Flying fish with a monoplane
body plan demonstrate different launching behaviors from their biplane
counterparts. Instead of extending their duration of thrust production,
monoplane fish launch from the water at high speeds at a large angle of attack (sometimes
up to 45 degrees). In this way, monoplane fish are taking advantage of their
adaptation for high flight speed, while fish with biplane designs exploit their
lift production abilities during takeoff.
Walking
Burrowing
Many fishes,
particularly eel-shaped fishes such as true eels, moray eels, and spiny eels,
are capable of burrowing through sand or mud. Ophichthids are capable of
digging backwards using a sharpened tail.
Reference
http://www.biology-resources.com/fish-01.html
http://en.wikipedia.org/wiki/Fish_locomotion
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